A red alga (division Rhodophyta), most of which are multicellular, the cells lacking flagella and centrioles, marine with about 200 species found in fresh water, and sexually reproducing. They contain the pigment phycoerythrin, which absorbs blue light and reflects red light, giving the algae their colour. They occur worldwide and there are up to 10,000 species. Some, such as nori, are edible.